This is very true. The human brain is basically a very efficient association machine fed by an incredibly powerful pattern matcher that builds connections even as it uses them. Knowledge you gain is never lost, but if you don't recall it often, it does not get associated with a lot of things. So the patterns you can feed your memory with to retrieve it are very exact, in other words, you will only remember it within a specific context. You can, however, with deliberate training improve your pattern matching ability and thus your ability to retrieve seldom needed facts from memory.

In this respect, the mind is like a muscle - the more you strain it, the more capable it gets.

This means it pays off to feed your memory stuff for two reasons. It will be there, even if you don't currently have the circumstances to recall it. And your matching abilities get better the more distantly related the facts you manage to associate are - for which broad and plentiful knowledge is a prerequisite. It's a feedback process.

Makeshifts last the longest.